The invention relates to a pre-treatment method for an electromechanical transducer, in particular for polarizing a piezoelectric actuator for an injection system of an internal combustion engine. Prior to the actual operation, the transducer is actuated with an electrical pre-treatment signal in order to set a predefined operating point in accordance with the pre-treatment signal.
In conventional common rail injection systems for internal combustion engines, electromechanical transducers in the form of piezoelectric actuators are used for controlling the injection of fuel into the combustion chambers of the internal combustion engine in the injectors of the injection system. The piezoelectric substrate of such piezoelectric actuators usually has a multiplicity of domains which are arranged spatially distributed and which to a certain extent have different orientations. Before the actual operation, the piezoelectric actuators are therefore polarized in order to align the individual domains within the piezoelectric substrate. For this purpose, the piezoelectric substrate is subjected to an electrical signal. A disadvantage with this is that during the subsequent normal operation of the piezoelectric actuators, continuous repolarization of the piezoelectric substrate takes place, as a result of which the operating point of the piezoelectric actuators is displaced, which leads to a changed actuation behavior.
U.S. Pat. No. 4,966,119 (European published patent application EP 0 324 450 A2) discloses a pre-treatment method for a piezoelectric actuator wherein, before the actual operation, the piezoelectric actuator is actuated with two so-called dummy pulses. However, this is not a pre-treatment method according to the invention, wherein the pre-treatment of the piezoelectric actuator is carried out at the factory. In contrast thereto, the pre-treatment of the piezoelectric actuator is carried out here directly before the internal combustion engine is started.
It is accordingly an object of the invention to provide a method for pre-treating an electro-mechanical transducer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and wherein a piezoelectric actuator is pre-treated so that a permanent setting of a predefined operating point is reached without the operating point subsequently being displaced during normal operation of the piezoelectric actuator.
With the foregoing and other objects in view there is provided, in accordance with the invention, a pre-treatment in method for an electromechanical transducer in a piezoelectric actuator for an injection system of an internal combustion engine, which comprises the following method steps:
prior to an actual operation of the electromechanical transducer, actuating the transducer with an electrical pre-treatment signal to set a predefined operating point in accordance with the pre-treatment signal;
and thereby setting the pre-treatment signal to a pulse sequence having a pulse length in a range between 0.1 ms and 2 ms, substantially corresponding with an electrical actuation signal during normal operation of the transducer, in order to prevent a displacement of a pre-set operating point during the normal operation of the transducer.
In other words, the invention comprises the general technical teaching of defining, within the scope of the pre-treatment process occurring before the actual operation, the electrical, mechanical, thermal and/or magnetic preferential conditions which occur during the actual operation.
The electromechanical transducer is preferably actuated during the treatment process with an electrical treatment signal which corresponds essentially to the electrical actuation signal during normal operation, in order to prevent a displacement of the pre-set operating point during normal operation. For this purpose, the pre-treatment signal is preferably composed essentially of a pulse sequence, the pulse frequency preferably being essentially equal to the actuation frequency during normal operation. Customary values for the pulse frequency thus lie between 10 and 120 Hz, an actuation frequency of 55 Hz being preferred. Values in the region of between 0.1 ms and 2 ms are preferable for the pulse duration of the pre-treatment signal, a value of 1.4 ms having proven particularly advantageous for the pulse duration of the pre-treatment signal.
During the actuation with the treatment signal, the transducer is preferably additionally mechanically loaded in order to simulate the normal operating state during the pre-treatment. For this purpose, the transducer is preferably pressurized with a force of 400 N to 1000 N, a prestressing force of 850 N having proven particularly advantageous.
In accordance with an added feature of the invention, the operating temperature of the transducer is additionally simulated in that, before the actual pre-treatment, the temperature of the transducer is changed to the normal operating temperature, which is usually between +50xc2x0 C. and +150xc2x0 C., a value of +80xc2x0 C. having proven particularly advantageous for the pre-treatment process.
In accordance with an additional feature of the invention, the response characteristics of the transducer are measured during the pre-treatment method in order to obtain information on the pre-treatment state of the transducer. The information which is acquired in this way can be used, for example, to adapt the pre-treatment parameters or to terminate the pre-treatment process after the desired state is reached. For this purpose, for example the electrical voltage, the electrical current, the travel and/or the electrical charge which has flowed through the transducer can be measured, but the invention is not restricted to the measurement of the abovementioned parameters.
In other words, the invention also comprises, during the actuation with the pre-treatment signal, measuring response characteristics of the transducer such as mechanical response characteristics and/or electrical response characteristics, to obtain information on a pre-treatment status of the transducer. Specifically, it is advantageous to measure an electrical voltage, an electrical current, a travel, and/or an electrical load that has flowed through the transducer to obtain information on the pre-treatment status. In a preferred implementation, the response characteristics of the transducer are compared with predefined reference values, and the pre-treatment process is terminated when one or more of the reference values are reached.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a pre-treatment method for an electromechanical transducer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.